Urea SNCR / aqueous-ammonia SNCR

Urea SNCR and aqueous-ammonia SNCR are two common reagent routes for selective non-catalytic reduction of NOx. Both inject a nitrogen-containing reagent into a hot furnace or boiler zone where it reacts with NOx without a catalyst. The useful temperature window is narrow, so injection location, droplet size, mixing and load following strongly affect performance.

The choice of reagent changes storage safety, dosing equipment, by-products and fouling risk.

Reagent comparison

Neither reagent works well if injected into gas that is too cold, too hot or poorly mixed.

Selection drivers

Selection depends on permit limits, reagent cost, safety rules, available space, load range, furnace temperature profile, existing ammonia infrastructure and the downstream sensitivity to ammonia slip. If an SCR, air heater or fabric filter sits downstream, ammonia slip can react with SO3 or acid gases to form sticky salts such as ammonium bisulphate.

Good SNCR systems use multiple injection levels, flow control, temperature feedback and regular tuning. Poorly tuned systems can miss NOx targets while increasing ammonia slip and fouling.

Acoustic-cleaning relevance

Sonic horns do not reduce NOx or replace reagent control. They can help where ammonium salts, urea decomposition products or fly ash accumulate on downstream heat-transfer surfaces, catalyst faces, ducts or hoppers. In Sylio-style reviews, SNCR reagent choice is recorded because it affects deposit stickiness and the likelihood of ammonium-salt fouling.

Selection factors

The choice between urea and aqueous ammonia is not only a reagent-price question. Urea is easier to transport and store in many jurisdictions, but it requires dissolution, pumping and thermal decomposition before the active reducing species are available in the furnace. Aqueous ammonia can react more directly, but it brings stronger odour, toxicity and handling controls. Both systems need the right temperature window, mixing, droplet size and residence time to reduce NOx without excessive ammonia slip.

Important variables include furnace temperature profile, load range, injection elevation, lance atomisation, carrier air or steam, reagent concentration, control response, flue-gas oxygen and baseline NOx. If injected too hot, reagent can oxidise back toward NOx. If injected too cold, reaction is incomplete and ammonia slip rises. Stratified gas flow can leave one side overtreated and the other side undertreated.

Fouling and safety implications

Ammonia slip from either reagent can react with SO3 to form ammonium bisulphate or related salts downstream. Those deposits can plug air preheaters, blind catalyst, foul ducts and create sticky ash that is difficult for acoustic cleaning to remove once aged. Operators should trend NOx, slip, load, injection rate, air-heater pressure drop and deposit condition together when diagnosing SNCR side effects.

Safety controls differ by reagent. Urea systems need housekeeping around solids handling, tank cleanliness, pump reliability and avoidance of blocked lances. Aqueous ammonia systems need ventilation, leak detection, personal protective equipment, bunding and emergency procedures suitable for the concentration used. Acoustic cleaning can support the downstream plant by limiting dry ash build-up, but it cannot compensate for poor SNCR temperature targeting or high ammonia slip.

Related terms

• Selective Non-Catalytic Reduction (SNCR)
• Ammonia slip
• NOx, SOx and CO
• Ammonium bisulphate